BOEING 757 Antenna Installation Bird Strike Substantiation

Airframe Designs was engaged by STC Twenty One (EASA Part21J DOA) to carry out a Bird Strike Substantiation assessment of a Cobham SATCOM HGA-7000 antenna installed on the forward fuselage crown on a BOEING 757-200 aircraft.

The objective was to convince both the FAA and EASA that the installation met the original certification basis requirements for discrete source damage per FAR/CS 25.571 (e)(1) i.e.

The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of Impact with a 4-pound bird at likely operational speeds at altitudes up to 8,000 feet.

The assessment identified critical features associated with the antenna itself and its attachment to the fuselage to identify the most likely points of structural failure.

An assessment was required to determine static strength Reserve Factors (RFs) for quasi-static strength and quantify the level of conservatism included in the assessment.

An empirical formula from RAE paper TR72056 was used to calculate the impact force acting on the antenna:

In addition to the strength calculations for the bird impact force, qualitative assessments also considered:

– The probability of bird strike based on the size and location of the antenna in relation to the aerodynamic boundary layer and flying attitude of the aircraft.
– Service history of the HGA-7000 in terms of number of installations, types of platform, and years in service.
– Antenna / fuselage materials, construction, means of attachment and level of redundancy in available load paths.
– Ability of the radome to experience a discrete penetration during impact but retain the ability to hold stagnation pressure.

“AFD provided structural specialists with experience in the application of the empirical formula for the assessment of bird impact forces. Extensive dialogue between AFD, Cobham, STC21, and the authorities (EASA and FAA) resulted in agreement on the Certification Plan and
proposed Means of Compliance, leading to a successful STC application”.